We all receive an unwelcome amount of spam. And the subject lines are often loud, aggressive and promotional.
The more of these spam subject lines people read, the more finely tuned their own sensitivity to spammy language becomes.

In other words, the final spam filter is the recipient of your email.

Be careful not to sound like spam in your subject line. Avoid that kind of language-- Free, Offer, Money, exclamation marks, misspelled words, ALL CAPS and the like.

Instead, go for recognition and then quietly describe the contents in a relevant, direct manner that shows how useful and helpful your email or newsletter is.

I know. This is often tough to do in 40 characters. But therein lies the challenge.